My AGM (absorbed glass mat) battery educated me yesterday. It's kind of a long story going back aways.
I've got AGM batteries in both the coupe & the roadster, but the coupe running fuel injection added to the issues. When I lost my wife to cancer 4 years back I pretty much lost interest in a lot of things, and sometimes went 3, 4, or 6 months without starting the coupe. It was on a battery tender, but when I did start it I'd have to crank it a bit and then it would have a couple of cylinders missing for a bit. If I took it out and let it run a ways it would smooth out, then it would sit for a few months again. About a month ago a friend was bringing his son & grandsons out so I backed the coupe out, ran it up to 3000 for a bit to smooth out and shut it down. Had the roadster out too, and an hour later they arrived and we spent some time looking at the cars. Lit the roadster off, then tried the coupe and it wouldn't fire. Just cranked. Later I hooked up jumpers and it lit off - first clue.
Monday I pulled the plugs and found them black & kind of wet, smelling of gas so a new set of plugs, oil & filter changed and I decided to take it for a spin. It had been on the battery tender, so it cranked good but didn't light off immediately - second or third try. Took it out and it ran OK, did a half hour or so to get it good and warm and blow any carbon out and came home. In the garage I left it off the charger, came back in 1/2 hour and it cranked strong but would not light off. I noted the hand held tuner was showing system voltage, and when I cranked it was down to 8.2Volts - second clue.
After thinking a bit I unhooked the ignition wire from the battery and used a pair of jumper wires to connect it to my ATV battery, hooking the two negatives together to complete the feed. Starter is from the AGM, ignition from the 4 wheeler. Hit the starter and it fired immediately - third clue and we're getting somewhere.
Talked to a kid at Batteries Plus and he suggested putting it on the charger, then bringing it in to be checked. First was the Battery Tender, then decided it needed the charger which has settings for standard, AGM and GEL batteries. After an hour at 3A it was still at 95% so I kicked it up to the Quick Charge. After an hour it was showing 100%, maintaining but when I lifted the lead and hooked it back it dropped to 95% again! After 15 minutes it was again 100%, but when I switched it to the 3A setting it again dropped back to 95%!
At Batteries Plus the kid put a volt meter on it and pronounced it had good voltage, but thankfully the manager, a guy around 50 had been listening to my tale and told him to put the battery under load. My 550 Cranking Amp battery had only 300Amps capacity. Worse, under load the terminal voltage dropped almost immediately. In the coupe it would start the first time, but it then took it a long time running on the alternator charge to get back to full capacity. With a new battery it cranked faster, and lit off quickly. I was low on fuel, so I decided to let it sit for a bit and if it started OK make a run to the gas station. The coupe started great, ran great, and after filling the tank lit off quicker than it has in years!
The lesson I learned is that an AGM battery may take a full charge and show 13.4V off the charger, but if the voltage falls off the battery is bad. They're not like a lead/acid unit where you lose a cell, they fail slowly. This one was 10 years old and gave every indication of being OK but it was failing. I've got a lot more faith in the old coupe now, and it was actually a hoot to drive for gas!
